Method and apparatus for testing the quality of fruit

ABSTRACT

A method of testing the quality of a fruit including applying a dynamic impact force to the fruit, detecting the mechanical response of the fruit to the dynamic force via a piezoelectric film transducer supported on a displaceable resilient base member such that the film transducer is bent by the dynamic impact force, to induce a strain in the film transducer, and the film transducer outputs an electrical signal corresponding to the change of the induced strain in the film transducer caused by the dynamic force and analyzing the electrical signal to indicate the quality of the fruit. Apparatus for carrying out the method is also described and claimed.

FIELD OF THE INVENTION

The present invention relates to methods and apparatus for testing thequality of fruit.

BACKGROUND OF THE INVENTION

High-value fresh agricultural products, particularly those intended forexport, must be carefully handled and sorted in order to meet highquality standards. Many methods are available for detecting quality andfor sorting according to external fruit properties, such as size, shape,color and external appearance. Internal properties, such as ripeness,taste, flavour, and internal damage, are generally determinedindirectly, by linking the property to one or more external fruitproperties, or are measured directly through destructive tests.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a novel non-destructivemethod and apparatus for testing the quality of a fruit.

According to the present invention, there is provided a method oftesting the quality of a fruit, comprising: applying a dynamic force tothe fruit; detecting the mechanical response of the fruit to saiddynamic force via a piezoelectric film transducer supported on adisplaceable supporting member such that the film transducer is bent, toinduce a strain therein, by said dynamic force and outputs an electricalsignal corresponding to the rate of change of the induced strain in thefilm transducer caused by said dynamic force; and analyzing saidelectrical signal to indicate the quality of the fruit.

A piezoelectric film transducer is a relatively new type of transducerwhich has begun to find extensive use in acoustical applications, bothin acoustical emitters and in acoustical receivers. One construction nowcommercially available includes a piezoelectric polymeric film ofpolyvinylidene fluoride having an electrically-conductive film coated onits opposite sides. The present invention utilizes such piezoelectricfilm transducers by supporting them on a displaceable supporting memberso that the film transducer is bent by the dynamic force (asdistinguished from a static force) to induce a strain in the filmtransducer. Since the induced strain varies with the rate of change ofthe applied dynamic force, the output of the film transducer will be ameasurement of the rate of change of the applied dynamic force. It isthis output which is analyzed to indicate the quality of the fruit.

According to further features of the invention as included in thepreferred embodiments described below, the dynamic force applied to thefruit, to produce the mechanical response detected by the piezoelectricfilm transducer, may be an impact force applied mechanically,electro-magnetically or pneumatically. The dynamic force may also be animpulse force or a vibrational force, such as applied mechanically byanother piezoelectric film transducer, a conventional piezoelectriccrystal transducer or a mechanical vibrator, or as applied acousticallyby a loudspeaker, etc. The dynamic force may also be an inertial forceapplied by accelerating or decelerating the fruit.

According to further features included in many of the describedpreferred embodiments, the displaceable supporting member may be aresilient base, a flexible beam, or a pivotal arm.

According to still further features in the described preferredembodiments, the mechanical response of the fruit which is detected andanalyzed may be the resonance frequencies of sonic waves produced in thefruit as a result of the dynamic force applied thereto, or may be theattenuation rate of sonic waves produced in the fruit as a result of thedynamic force applied thereto.

The invention also provides novel apparatus for testing the quality of afruit in accordance with the above methods.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, somewhat diagrammatically and by wayof example only, with reference to the accompanying drawings, wherein:

FIG. 1 illustrates one form of apparatus for testing the quality of afruit in accordance with the present invention;

FIG. 2 illustrates a second form of apparatus for testing the quality ofa fruit in accordance with the present invention;

FIGS. 3A, 3B, 3C and 3D illustrate time and frequency responses of anapple tested in accordance with the invention;

FIG. 4 illustrates the frequency response of several fruit varietiestested in accordance with the present invention;

and FIGS. 5, 6, 6A, 7, 8, 9, 10, 11, 12, 12A, 13 and 14 illustratefurther forms of apparatus for testing the quality of a fruit inaccordance with the present invention, FIGS. 6a and 12a being top planviews of FIGS. 6 and 12, respectively.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The apparatus illustrated in FIG. 1 comprises a fruit bed 2 for holdingthe fruit F, such as an apple, to be tested. The fruit bed 2 is equippedwith one or more displaceable contact elements engaging the outersurface of the fruit F when received within the fruit bed. In theexample illustrated, there are three such displaceable contact elements4, 6, 8, thereby providing three points of contact with the outer lowersurface of the fruit. Each contact element 4, 6, 8, includes apiezoelectric film transducer 10, 12, 14 in direct contact with theouter surface of the fruit F, and a resilient base or pad 16, 18, 20pressing the piezoelectric film transducer into firm contact with thefruit.

The illustrated apparatus further includes an arm 22 pivotally mountedwith respect to the fruit F within the fruit bed 2, and carrying afurther displaceable contact element 24 for engagement with the outerupper surface of the fruit F. Displaceable contact element 24 includes apiezoelectric transducer 26, of the film type or of the crystal type,and a spring 28 serving as a resilient urging the transducer into firmcontact with the outer surface of the fruit F. Arm 22 is pivotallymounted at 30 to bring contact element 24 into or out of engagement withthe fruit F.

The piezoelectric transducer 26 engaging the upper surface of the fruitF serves as an exciter or transmitter for transmitting a dynamic force,e.g., a mechanical impulse force or a sonic vibrational force, to thefruit F; whereas the piezeoelectric film transducers 10, 12, 14 engagingthe lower surface of the fruit serve as receivers for detecting themechanical response of the fruit F to the applied dynamic force. Sincethe exciter transducer 26 applies a dynamic force, it may be of thepiezoelectric film type or of the conventional crystal type. Thedetector transducers 10, 12, 14 are of the piezoelectic film type, andsince they are supported on their respective resilient bases, thedynamic forces propagated through the fruit cause them to bend to inducestrains in their respective film transducers, corresponding to theapplied dynamic force. The piezoelectric film transducers 10, 12, 14output electrical signals corresponding to the rate of change of thestrains induced in them. These electrical signals are analyzed toindicate the quality of the fruit F under test.

Thus, the system illustrated in FIG. 1 includes an exciting signalgenerator 40 under the control of a control unit 42 for exciting the toptransmitter piezoelectric transducer 26, to generate the dynamic force,e.g., a mechanical impulse force or sonic vibrational force, applied tothe fruit F. The electrical signals generated by the receiverpiezoelectric film transducers 10, 12, 14 are fed to a signalconditioning circuit 44 before being applied to an analog-to-digitalconverter 46. The latter circuit converts the analog output of thereceiver transducers 10, 12, 14 to digital form, for analysis in thesignal analysis and quality determination circuit or module 48. Theoutput of the signal conditioning circuit 44 is also applied to amonitor/oscilloscope 50 for direct viewing.

It will be appreciated that only one piezoelectric film transducer,e.g., transducer 10 in FIG. 1, is needed to detect the mechanicalresponse of the fruit to the dynamic force applied to it. It will alsobe appreciated that more than one transducer may be used as atransmitter, e.g., transducer 14 at the bottom of the fruit. It will befurther appreciated that the measuring and control circuitry illustratedin FIG. 1 could be partly or completely embodied in a microprocessorrather than in discrete circuitry.

The piezoelectric film transducers in 10, 12, 14 (and also 26 if a filmtransducer is used) are commercially available polyvinylidene fluoridepiezoelectric film coated on opposite sides with a conductive coating,such as of silver.

FIG. 2 illustrates apparatus similar to that of FIG. 1, and tofacilitate understanding, the corresponding parts are identified by thesame reference numbers. In the apparatus of FIG. 2, however, the fruit Funder test is subjected to an impact force. For this purpose, thetransmitter piezoelectric transducer 26 is omitted and is replaced by animpact element in the form of a pendulum 60 which applies apredetermined impact force against the fruit F under test. The systemillustrated in FIG. 2 further includes a light source 62 and aphotodiode 64 for detecting the instant just before the fruit F isimpacted by the pendulum 60.

In the apparatus illustrated in FIG. 2, the impact element 60 ismechanically applied by a small wooden rod pendulum. The instant ofimpact, or just before impact, by the pendulum 60 is detected byphotodiode 64 connected to the control unit 42. The signal so producedby the photodiode 64 is used to trigger the dynamic sensor systemincluding the piezoelectric film transducers 10, 12, 14. While FIG. 2illustrates the instant just before impact, the point of impact beingdetected by the photodiode 64, it will be appreciated that this couldalso be detected by the change in the output of one or all of thepiezoelectric film transducers 10, 12, 14.

Various varieties of fruit were tested by the apparatus illustrated inFIG. 2. FIG. 3 illustrates an example of the recorded response of anapple in time domain before and after high-pass filtering of 300 Hz(FIGS. 3a and 3b) and their Fast Fourier Transforms (FFTs, FIGS. 3c and3d, repectively). These figures show that the high resonance frequenciesof fruit response may be super-imposed on a basic, low frequency signalthat may be caused by the fruit bed 2. The higher frequencies representthe real natural frequencies of the tested fruit.

The low resonance frequency of the tested fruit and the spring constantof the fruit also relate to the mass of the body, and thereforemeasuring the low resonance frequency of the fruit may also be used fordetermining the weight of the fruit according to known techniques.

FIG. 4 illustrates the decline of the first resonance frequencies ofvarious fruits during eight days. These tests showed a decrease of 1.4%to 5.5% per day in the resonance frequency of the various fruits,thereby showing that the resonance frequency provides an indication ofthe freshness of the fruit.

FIGS. 5-14 illustrate other arrangements in which the fruit may beexcited, and its mechanical response may be detected, to provide anindication of the quality of the fruit.

Thus, FIG. 5 illustrates an arrangement wherein the fruit F is receivedin a bed 50 and is impacted by a pendulum 51, while its mechanicalresponse is detected by a piezoelectric film transducer 52 supported ona resilient base 53 on which the fruit F rests. A side wall of bed 50 isprovided with a resilient damping element 54 which may also include apiezoelectric film transducer 55 on its outer surface in contact withthe fruit F. The resilient bases 53 and 54 may be soft rubber pads orthe like. The arrangement illustrated in FIG. 5 may also include a lightsource and a photodiode (not shown) to detect the instant before impactof the pendulum 51 with the fruit F, or may detect the instant of impactby the output of the transducer 55.

FIGS. 6 and 6a illustrate another arrangement including a bed 60 forreceiving the fruit F, in which the dynamic force applied to the fruitis effected by a pneumatically-actuated impact element 61. The responseto the impact is detected by three piezoelectric film transducers 62a,62b, 62c, each supported on a resilient base or pad 63. The bed 60 isformed with a triangular cavity 64, whose floor is formed with recesses64a, 64b, 64c extending between a common intersection point 65 and thethree corners of the cavity. The three piezoelectric film transducers62a-62c extend from the common intersection point 65 to one of the sidesof the triangle. In such an arrangement the fruit F being tested iscontacted only at three points by the piezoelectric film transducers.

FIG. 7 illustrates another arrangement wherein the fruit bed 70receiving the fruit F being tested is impacted by anelectromechanically-actuated impact element 71; and the mechanicalresponse to the impact is detected by a plurality of piezoelectric filmtransducers 62a, 62b, each supported on a resilient base 63a, 63b.Although FIG. 7 illustrates two such detectors, there would preferablybe three such detectors arrayed as shown in FIG. 6. The same applieswith the other arrangements described below.

FIG. 8 illustrates an arrangement including a fruit bed 80 receiving thefruit F to be tested, wherein the dynamic force applied to the fruit isa vibrational acoustical force applied by a loudspeaker 81. Themechanical response of the fruit to the applied force is detected bypiezoelectric film transducers 82a, 82b each supported on a resilientbase 83a, 83b, which may be arranged as described above with respect toFIGS. 6 and 7. In addition, the arrangement illustrated in FIG. 8includes a further pair of piezoelectric film transducers 64a, 64b,mounted on the opposite sides (or on only one side) of an elastic beam65, such as a leaf spring, biassed against the side of the fruit F beingtested.

FIG. 9 illustrates a further arrangement including a fruit bed 90 inwhich the fruit F to be tested is excited by a piezoelectric crystaltransducer 91, and the mechanical response of the fruit is detected by apiezoelectric film transducer 92 supported on a resilient base 93.

FIG. 10 illustrates an arrangement including a bed 100 in which thefruit F to be tested is excited by a pair of piezoelectric filmtransducers 101a, 101b, mounted on opposite sides of an elastic beam inthe form of a leaf spring 102; and the mechanical response of the fruitto the excitation is detected by another pair of piezoelectric filmtransducers 103a, 103b, mounted on the opposite sides (or on only oneside) of another elastic beam 104.

FIG. 11 illustrates a further arrangement including a bed 110 in whichthe fruit F to be tested is excited by a pair of piezoelectric elements111a, 111b (e.g., crystals or films) supporting the fruit, and in whichthe mechanical response of the fruit to the excitation is detected by afirst pair of piezoelectric film transducers 112a, 112b, mounted onopposite sides (or on only one side) of an elastic beam, e.g., a leafspring, 113 and urged thereby into contact with one side of the fruit F,and a second pair of piezoelectric film tranducers 114a, 114b, mountedon opposite sides (or on only one side) of an elastic beam 115 and urgedagainst the opposite side of the fruit.

FIGS. 12 and 12a illustrate a further arrangement including a bed 120,in which the fruit F being tested is excited by a piezoelectric crystaltransducer 121, and the mechanical response thereto is detected by apiezoelectric film transducer 122 supported on the outer face of aresilient pad 122a carried on one side of an arm 123 pivoted at 124 andurged into contact with the fruit F by a spring 125. The opposite sideof bed 120 may carry a second piezoelectric film transducer 126supported on a resilient pad 127.

FIG. 13 illustrates a further arrangement including a bed 130 in whichthe dynamic force applied to the fruit F for exciting it is an inertialforce produced by the sudden acceleration and/or deceleration of thefruit. In the illustrated arrangement, the bed is driven in thedirection of the arrow 131 and engages a cam 132 having an upwardlyinclined face 132a and vertical face 132b engaged by a cam follower 133,which decelerates the bed in the upward direction, followed by a sharpdrop which more quickly accelerates the bed and fruit in the oppositedirection, to apply inertial forces to the fruit F. The response of thefruit to the inertial forces so applied is detected by a pair ofpiezeoelectric film transducers 134a, 134b, each mounted on a resilientpad 135a, 135b.

FIG. 14 illustrates an arrangement including a fruit bed 140 in whichthe excitation of the fruit F is effected by a pair of piezoelectricforce transducers 141a, 141b, each mounted on a resilient pad 142a,142b. Both are carried by a head 143 movable by a cylinder 144 towardsand away from the fruit bed 140. Head 143 may also be moved in the samedirection as the fruit bed, if the fruit bed is a travelling one. Theresponse to the force so applied to the fruit F is detected by a pair ofpiezoelectric film transducers 145a, 145b each supported on a resilientpad 146a, 146b.

While the invention has been described with respect to several preferredembodiments, it will be appreciated that many variations may be made.For example, in the arrangement illustrated in FIG. 14, all thetransducers could be mounted on the head 143. Many other variations,modifications and applications of the invention will be apparent.

We claim:
 1. A method of testing the quality of a fruit,comprising:applying a dynamic impact force to the fruit; detecting themechanical response of the fruit to said dynamic force via apiezoelectric film transducer directly mounted onto, and flush with, atop surface of a displaceable resilient pad, which is in turn mounted ona rigid base such that the film transducer is bent by said dynamicimpact force, to induce a strain in said film transducer, and said filmtransducer outputs an electrical signal corresponding to the change ofthe induced strain in the film transducer caused by said dynamic force;and analyzing said electrical signal to indicate the quality of thefruit.
 2. The method according to claim 1, wherein said mechanicalresponse is the resonance frequencies of sonic waves produced in thefruit as a result of the dynamic force applied thereto.
 3. Apparatus fortesting the quality of a fruit, comprising:a holder for receiving thefruit to be tested; a dynamic force impactor for applying a dynamicforce to the fruit in said holder; a piezoelectric film transducerdirectly mounted onto, and flush with, a top surface of a displaceableresilient pad, which is in turn mounted on a rigid base such that thefilm transducer is bent, to induce a strain in said film transducer, bysaid dynamic force and said film transducer outputs an electrical signalcorresponding to the change of the induced strain in the film transducercaused by said dynamic impact force; and an analyzer for analyzing saidelectrical signal outputted by the piezoelectric film transducer toindicate the quality of the fruit.
 4. The apparatus according to claim3, wherein said analyzer analyzes the resonance frequencies of sonicwaves produced in the fruit as a result of the dynamic force appliedthereto.